Process for producing steel product and production facility therefor

ABSTRACT

The present invention provides a method of manufacturing a steel product, comprising the step of heat treating a steel product having been subjected to quenching or accelerated cooling on a hot rolling line after hot rolling to pass the steel product through a plurality of induction heating apparatuses, which are installed on the hot rolling line, three times or more. The method of the present invention makes it possible to uniformly heat treat the steel product with high productivity.

This application is the United States national phase application ofInternational Application PCT/JP2003/009959 filed Aug. 5, 2003.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a steelproduct, in which the steel product having been subjected to quenchingor accelerated cooling after hot rolling is subjected to on-lineheat-treatment, in particular, to a method for manufacturing a steelproduct using a plurality of induction heating apparatuses, andmanufacturing facilities therefor.

BACKGROUND ART

In many cases, steel plates having a thickness of 8 mm or more arerapidly cooled by quenching or accelerated cooling after hot rolling andthen subjected to tempering treatment in order to achieve high strengthand toughness.

In recent years, while the quenching or the accelerated cooling iscarried out in an on-line manner, the tempering treatment is carried outin a gas combustion furnace as usual in an off-line manner, so that ittakes time to considerably decrease the productivity of steel plate.Therefore, several methods have been proposed to increase theproductivity in the tempering treatment process.

To increase the productivity, for example, JP-A-9-256053 proposes a newheating pattern of tempering treatment. In this heating pattern, thesteel plate is continuously transferred in a gas combustion furnace,which is set to high temperature at the inlet side thereof, and to lowtemperature at the outlet side thereof, to be subjected to heattreatment. To be concrete, the temperature at the inlet side of thefurnace is set to be higher by at least 200° C. than the target heattreatment temperature so that the furnace temperature is decreasedstepwise toward the outlet side of the furnace, and the temperature atthe outlet side of the furnace is set to be within ±20° C. from thetarget heat treatment temperature. In a heating system by gascombustion, however, the heat transfer is conducted through radiationand convection, so that the rapid heating is impossible and theproductivity can not be adequately increased.

JP-A-4-358022 and JP-A-6-254615 propose, as a heat treatment method forincreasing the productivity, an on-line heat treatment method, in whicha heating apparatus is installed on a rolling line to heat the steelplate. The former discloses a method for manufacturing a steel platehaving high strength and toughness, in which a rolling mill, anaccelerated cooling apparatus, and a heating apparatus are arranged on arolling line to perform rapid heating in tempering treatment, and thelatter discloses a method, in which a rolling mill, a leveler, anaccelerated cooling apparatus, and a″heat keeping furnace are arrangedon a rolling line so that the residual stress in steel plate, generatedby rolling and accelerated cooling, is removed in the heat keepingfurnace. However, these methods cause problems that it takes time forheat treatment and that the steel plate can not be uniformly subjectedto heat treatment.

JP-A-48-25239 discloses a method for heat treating a steel plate, inwhich a plurality of solenoid type induction heating apparatuses arearranged in series on a rolling line. When the steel plate is heated tothe target temperature with a single induction heating apparatus, theinduced current is localized in the vicinity of the steel plate surface,thus sometimes causing a problem that the surface thereof is excessivelyheated to over the Curie point or the Ac1 transformation point.Therefore, since it is difficult to heat the thickness-wise center ofsteel plate to the target temperature with a single induction heatingapparatus during controlling the surface temperature of steel plate toor below a certain temperature, JP-A-48-25239 arranges two or moreinduction heating apparatuses in series, and passes a steel platethrough these apparatuses so that the surface temperature of the steelsheet does not exceed the upper limit temperature and the thickness-wisecenter temperature reaches the target temperature. For that purpose,however, it is necessary to decrease the transfer speed of steel plate,so that it is not possible to attain the adequately high productivity.In particular, in case of the steel plate having a large thickness, ittakes time for heating the thickness-wise center of the steel plate, andtherefore the transfer speed has to be decreased to a large extent, sothat the productivity is remarkably decreased.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method formanufacturing a steel product, in which the steel product can beuniformly subjected to heat treatment with high productivity usinginduction heating apparatuses arranged on a hot rolling line, andmanufacturing facilities therefor.

The above object is attained by the following methods.

1) A method of manufacturing a steel product, comprising the step ofheat treating a steel product having been subjected to quenching oraccelerated cooling on a hot rolling line after hot rolling by passingthe steel product through a plurality of induction heating apparatuses,which are installed on the hot rolling line, three times or more. Here,one time of passage means that the steel product passes through aplurality of induction heating apparatuses once in one direction.Accordingly, when the steel product goes and comes back through theinduction heating apparatuses once, the number of times of passageamounts to two.

2) A method of manufacturing a steel product, comprising the step ofheat treating a steel product having been subjected to quenching oraccelerated cooling on a hot rolling line after hot rolling by passingthe steel product through a plurality of induction heating apparatuses,which are installed on the hot rolling line, at least once, and whereinthe number of times of passage through the induction heating apparatusesis such a number of times that the surface temperature and thethickness-wise center temperature of the steel product fall in apredetermined temperature range in the shortest time.

3) A method of manufacturing a steel product, comprising the step ofheat treating a steel product having been subjected to quenching oraccelerated cooling on a hot rolling line after hot rolling by passingthe steel product through a plurality of induction heating apparatuses,which are installed on the hot rolling line, at least once, and whereinthe number of times of passage through the induction heating apparatusesis such a number of times that the surface temperature and thethickness-wise center temperature of the steel product fall in apredetermined temperature range within a target treatment time.

4) A method of manufacturing a steel product, comprising the step ofheat treating a steel product having been subjected to quenching oraccelerated cooling on a hot rolling line after hot rolling by passingthe steel product through a plurality of induction heating apparatuses,which are installed on the hot rolling line, at least once, and whereinthe steel product is subjected to heat treatment so that heat treatmenttime, which is calculated on the basis of the dimensions and thenecessary temperature rise of the steel product, the number of times ofpassage through the induction heating apparatuses, and the heatingcapacities of the induction heating apparatuses, and which elapses untilthe surface temperature of the steel product does not exceed apredetermined upper limit temperature and the temperature in apredetermined position inside the steel product reaches a targettemperature, falls within a target treatment time.

5) A method of manufacturing a steel product, comprising the step ofheat treating a steel product having been subjected to quenching oraccelerated cooling on a hot rolling line after hot rolling by passingthe steel product through a plurality of induction heating apparatuses,which are installed on the hot rolling line, at least once, and whereinthe steel product is subjected to heat treatment so that heat treatmenttime, which is calculated on the basis of the dimensions and thenecessary temperature rise of the steel product, the number of times ofpassage through the induction heating apparatuses, and the heatingcapacities of the induction heating apparatuses, and which elapses untilthe surface temperature of the steel product does not exceed apredetermined upper limit temperature and the temperature in apredetermined position inside the steel product reaches a targettemperature, becomes the shortest.

6) A method of manufacturing a steel product, comprising the step ofheat treating a steel product having been subjected to quenching oraccelerated cooling on a hot rolling line after hot rolling by passingthe steel product through two to five induction heating apparatuses,which are installed on the hot rolling line, three times or more.

These methods can be realized by manufacturing facilities of steelproduct comprising, on a hot rolling line, a hot rolling mill, aquenching or accelerated cooling apparatus, a plurality of inductionheating apparatuses, and an operating unit for heat treatment pattern ofthe induction heating apparatuses, and wherein the operating unitcomprises means to calculate scheduled time, at which a steel producthaving been subjected to quenching or accelerated cooling after hotrolling reaches the induction heating apparatuses, and means todetermine a heat treatment pattern, which prevents a succeeding steelproduct to be subjected to heat treatment from waiting on the hotrolling line, from the dimensions and the necessary temperature rise ofthe steel product, and scheduled time, at which the succeeding steelproduct reaches the induction heating apparatuses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of manufacturing facilities of steel productaccording to the present invention.

FIG. 2 is a detail of the induction heating apparatus shown in FIG. 1.

FIGS. 3A to 3C are different heat treatment patterns depending on thenumber of induction heating apparatus and the number of times ofpassage.

FIG. 4 is a number of times of passage preferable for heat treatmenttime.

FIG. 5 is a number of times of passage preferable for electric powerconsumption rate.

FIG. 6 is another example of manufacturing facilities of steel productaccording to the present invention.

FIG. 7 is a detail of the reheating furnace control computer shown inFIG. 6.

FIG. 8 is a detail of the rolling control computer shown in FIG. 6.

FIG. 9 is a detail of the cooling control computer shown in FIG. 6.

FIG. 10 is a detail of the operating unit shown in FIG. 6.

FIG. 11 is a flowchart of operation.

FIG. 12 is a detail of the flowchart of operation.

FIG. 13 is a detail of the flowchart of operation.

FIG. 14 is a flowchart for calculation of scheduled time, at which thesucceeding steel product is cooled.

EMBODIMENTS OF THE INVENTION

FIG. 1 shows an example of manufacturing facilities of steel productaccording to the present invention.

The steel product 2 rolled by a hot rolling mill 1 is subjected toquenching by a water cooling apparatus 3, corrected in shape by aleveler 4, and then subjected to heat treatment by a plurality ofinduction heating apparatuses 5. As the induction heating apparatuses 5,transverse type ones or solenoid type ones are applicable. However, thesolenoid type ones are preferable in view of controlling the temperaturein the vicinity of the surface of the steel product 2. In addition, theleveler 4 is not necessarily arranged after the water cooling apparatus3 but may be arranged upstream of the water cooling apparatus 3 ordownstream of the induction heating apparatuses 5 while it is desiredthat the leveler 4 be arranged at the inlet side of the inductionheating apparatuses 5 in view of uniformly heating the steel product 2and preventing the collision between the steel product 2 and theinduction heating apparatuses 5.

FIG. 2 shows a detail of the induction heating apparatuses shown in FIG.1.

The induction heating apparatus comprise a plurality of inductionheating apparatuses 5, a temperature sensor 6 provided at the inlet ofthe first induction heating apparatus 5 to detect the temperature of thesteel product 2, transfer rollers 7 for transferring the steel product2, a speed sensor 8 to detect the transfer speed of the steel product onthe basis of rotation of the transfer rollers 7, a controlling unit 9 tocalculate the electric power supplied to each induction heatingapparatus 5, a power supplying unit 10 to control the electric powersupplied to each induction heating apparatus 5 on the basis of theoutput from the controlling unit 9, and a temperature sensor 11 providedat the outlet of the last induction heating apparatus 5 to detect thetemperature of the steel product 2 after heating.

Embodiments of a method of manufacturing a steel product using theabove-described induction heating apparatuses, according to the presentinvention, will be described below.

First Embodiment

A steel product is subjected to so-called reverse heating, in which thesteel product goes and comes back through the induction heatingapparatuses three times or more. Since increasing the number of times ofpassage corresponds apparently to increasing the number of inductionheating apparatuses, the temperature rise every induction heatingapparatus can be reduced. The transfer speed of steel product can,therefore, be increased as compared with the case where the number oftimes of passage is set to one. Also, in the case where the reverseheating is carried out three times, for example, the steel product maybe heated only at the first and the last times without heating at thesecond time. In this case, since the transfer speed can be increased atthe second time to shorten the heat treatment time; it is possible toenhance the productivity.

Second Embodiment

A steel product is subjected to heat treatment in such a number of timesof passage that both the surface temperature and the thickness-wisecenter temperature fall in a predetermined temperature range in theshortest time.

Third Embodiment

A steel product is subjected to heat treatment by setting a number oftimes of passage through the induction heating apparatuses to thatnumber of times of passage, in which the heat treatment time, determinedby the relationship established between the number of times of passage,the transfer speed of the steel product and the electric power of theinduction heating apparatuses using the dimensions and the necessarytemperature rise of the steel product, becomes the shortest.

Namely, the steel product is subjected to heat treatment in such anumber of times of passage that the heat treatment time, in which thesurface temperature and the thickness-wise center temperature of thesteel product determined using the number of times of passage, thetransfer speed of the steel product and the electric power of theinduction heating apparatuses fall within a predetermined temperaturerange, becomes the shortest.

The procedure of determining the number of times of passage, thetransfer speed of the steel product each time, and the electric power ofeach respective induction heating apparatus conceivably includes thefollowing two methods, that is, (1) a method of determining the transferspeed, the number of times of passage, and the electric power everysteel product, and (2) a method of predetermining the number of times ofpassage, the transfer speed, and the electric power according to thedimensions of the steel product.

(1) The method of determining the transfer speed, the number of times ofpassage, and the electric power of the induction heating apparatusesevery steel product

[1] The dimensions and the necessary temperature rise of the steelproduct are acquired.

The thickness and the width of the steel product being subsequentlysubjected to heat treatment, and the conditions of target temperature,the upper limit temperature, etc. are acquired from a computer, whichcontrols the production.

[2] The transfer speed and the electric power are found in the casewhere the number of times of passage is one.

Assuming that the heat treatment is carried out once, the optimizationproblem is solved, in which the variables include the transfer speed ofthe steel product and the electric power of each induction heatingapparatus, the constraint conditions include the upper limit temperatureand the target temperature, and the objective function includes the heattreatment time and the amount of consumed electricity. In this case, theproblem can be solved using the optimization technique such as linearprogramming, non-linear programming, or the like, and can be also solvedby suitably changing the respective variables to find that thecombination of the transfer speed and the electric power, in which theheat treatment time becomes the shortest and the power consumptionbecomes the lowest.

[3] The transfer speed and the electric power are found in the casewhere the number of times of passage is three.

Assuming that the heat treatment is carried out three times, theoptimization problem is solved, in which the variables include thetransfer speed of the steel product each time and the electric power ofeach induction heating apparatus, the constraint conditions include theupper limit temperature and the target temperature, and the objectivefunction includes the heat treatment time and the amount of consumedelectricity. In this case, the problem can be solved using theoptimization technique such as linear programming, non-linearprogramming, or the like, and can be also solved by suitably changingthe respective variables to find that the combination of the transferspeed and the electric power, in which heat the treatment time becomesthe shortest and the power consumption becomes the lowest.

[4] The transfer speed and the electric power are found in the casewhere the number of times of passage is more than

Increasing the number of times of passage to five, seven, and so on inthe same manner as in [3], the combination of the transfer speed and theelectric power each time is found. The maximum number of times ofpassage is predetermined according to the dimensions and the temperaturerise of the steel product, and the processing in [4] is performed untilthe predetermined number of times is reached.

[5] The number of times of passage is determined.

Selecting the number of times of passage, in which the heat treatmenttime becomes the shortest, the steel product is subjected to heattreatment by using the transfer speed and the electric power at thattime.

(2) The method of predetermining the number of times of passage, thetransfer speed, and the electric power of the induction heatingapparatuses according to the dimensions of the steel product.

A table as shown in TABLE 1 and including the number of times of passageand the transfer speed according to the dimension of the steel product,is beforehand prepared every steel type and every heat treatmentpattern.

A table is prepared with respect to the conditions of heat treatment,for example, the following two conditions of heat treatment, that is,respective conditions of the initial temperature prior to the heattreatment, the target temperature, and the temperature rise being adifference therebetween. TABLE 1 is an example prepared on the basis ofthe conditions of heat treatment a).

a) Initial temperature: 400° C., target temperature: 600° C.,temperature rise: 200° C.

b) Initial temperature: 100° C., target temperature: 600° C.,temperature rise: 500° C.

TABLE 1 is prepared in the following processes [1] to [5].

[1] The dimensions and the necessary temperature rise of the steelproduct to be subjected to heat treatment are determined.

[2] The transfer speed and the electric power of each induction heatingapparatus in the case where the number of times of passage is one arefound.

Assuming that the heat treatment is carried out once, the optimizationproblem is solved, in which the variables include the transfer speed ofthe steel product and the electric power of each induction heatingapparatus, the constraint conditions include the upper limit temperatureand the target temperature, and the objective function includes the heattreatment time and the amount of consumed electricity. In this case, theproblem can be solved using the optimization technique such as linearprogramming, non-linear programming, or the like, and can be also solvedby suitably changing the respective variables to find that thecombination with electric power, in which the power consumption isminimized.

[3] The transfer speed of the steel product and the electric power ofeach induction heating apparatus in the case where the number of timesof passage is three are found.

Assuming that the heat treatment is carried out three times, theoptimization problem is solved, in which the variable is the electricpower of each induction heating apparatus, the constraint conditionsinclude the upper limit temperature and the target temperature, and theobjective function includes the amount of consumed electricity. In thiscase, the problem can be solved using the optimization technique such aslinear programming, non-linear programming, or the like, and can be alsosolved by suitably changing the respective variables to find that thecombination with the electric power, in which the power consumption isminimized.

[4] The transfer speed of the steel product and the electric power ofeach induction heating apparatus in the case where the number of timesof passage is more than three are found.

Increasing the number of times of passage to five, seven and so on inthe same manner as in [3], the combination of the transfer speed and theelectric power each time is found. The maximum number of times ofpassage is predetermined according to the dimensions and the temperaturerise of the steel product, and the processing in [4] is performed untilthe predetermined number of times is reached.

[5] The number of times of passage is determined.

The number of times of passage, in which the heat treatment time becomesthe shortest, and the transfer speed of steel product are determined.Though not shown in TABLE 1, the electric power at that time is alsoobtained.

When the heat treatment is actually carried out, the number of times ofpassage and the transfer speed given in TABLE 1 are selected accordingto the steel type, the conditions of the heat treatment, and thedimensions of the steel product.

At that time, the actual initial temperature is measured, and when it isdifferent from the assumed initial temperature, the electric power iscorrected based thereon.

TABLE 1 WIDTH (mm) 1000 2000 3000 NUMBER OF NUMBER OF NUMBER OFTHICKNESS TIMES OF SPEED SPEED SPEED TIMES OF SPEED SPEED SPEED TIMES OFSPEED SPEED SPEED (mm) PASSAGE 1 2 3 PASSAGE 1 2 3 PASSAGE 1 2 3 10 1 601 40 1 20 20 1 40 1 20 3 30 40 40 30 1 20 3 30 40 40 3 20 30 30 SPEED 1,2 and 3: transfer speed (m/min)

Fourth Embodiment

In the case where the reverse heating is to be carried out with thenumber of times of passage being three or more, the transfer speed ofthe steel product is changed every time. As described in the thirdembodiment, the changing of the transfer speed every time is effectivein order to meet the temperature restrictions and to minimize the heattreatment time and the power consumption.

Fifth Embodiment

In the case where the reverse heating is to be carried out with thenumber of times of passage being n not less than three, the heattreatment time is shortened by making the transfer speed of the steelproduct in the nth passage and the (n-1)th passage larger than thetransfer speed of the steel product prior to the (n-2)th passage.

In case of carrying out the heat treatment in, for example, three times,the transfer speed of the steel product is set to the transfer speed ofthe first passage<that of the second passage and the transfer speed ofthe first passage<that of the third passage. Since the temperature ofthe steel product is raised by the first heat treatment, it is possibleto increase the transfer speed at the second passage and the thirdpassage, so that the heat treatment time can be shortened and the powerconsumption can be reduced as compared with the case where the heattreatment is carried out at the same transfer speed for all thepassages.

Sixth Embodiment

First, the basic concept of this embodiment will be described in thefollowing.

To heat treat the steel product having been subjected to quenching oraccelerated cooling after hot rolling by passing the steel productthrough the induction heating apparatus arranged on a hot rolling line,there seems be two methods. One is to carry out a heat treatment inwhich the number of times of passage is one and the number of inductionheating apparatus is plural, and the other is to carry out a reverseheating in which the number of induction heating apparatus is one, thenumber of times of passage is plural, and the steel product goes andcomes back through the induction heating apparatus.

The heat treatment time was compared between the following cases (A) to(C), in which the steel product having a thickness of 25 mm, a length of25 mm, and a width of 3.5 m was subjected to heat treatment withrestrictions of the starting heat temperature being 450 r, the surfaceupper limit temperature being 710° C., and the target temperature being650° C.

(A) Number of induction heating apparatus: 6, number of times ofpassage: 1

(B) Number of induction heating apparatus: 3, number of times ofpassage: 1

(C) Number of induction heating apparatus: 3, number of times ofpassage: 3

The optimum transfer speed of the steel product and the electric powerof the induction heating apparatus, which met the above temperaturerestrictions, were calculated for the respective three cases. Thefollowing result was obtained.

(A) Transfer speed: 55 m/min, electric power consumption rate: 56.6kWh/ton

(B) Transfer speed: 15 m/min, electric power consumption rate: 50.8kWh/ton

(C) Transfer speed: 50 m/min at the first passage, 120 m/min at thesecond passage, 120 m/min at the third passage, electric powerconsumption rate: 55.6 kWh/ton

FIG. 3 shows heat treatment patterns for the surface temperature, thethickness-wise center temperature, and the average temperature of thesteel product under the above conditions. FIG. 3A shows results in thecase (A), FIG. 3B shows results in the case (B), and FIG. 3C showsresults in the case (C). Here, the temperature is measured at the topend of the steel product. Also, the period of time, in which the surfacetemperature rises and falls in a short time (around 5 seconds) to form apeak, corresponds to the period, in which the top end of steel productpasses through the induction heating apparatuses. Six peaks appear inthe case of six induction heating apparatuses and one time of passage inFIG. 3A, three peaks appear in the case of three induction heatingapparatuses and one time of passage in FIG. 3B, and three peaks appearthree times in the case of three induction heating apparatuses and threetimes of passage in FIG. 3C. The reason why there is a large timeinterval between three peaks indicative of the first passage and threepeaks indicative of the second passage is that the temperature of theleading end of the steel product is measured as described above and along period of time lapses until the trailing end of the steel productgoes out from the first passage and then the leading end of the steelproduct goes in the second passage. In addition, the surface temperatureis controlled so that its peak value does not exceed the Curie point andthe Ac1 transformation point, allowing the steel product to have desiredproperties, for example, hardness and toughness.

In comparison among FIGS. 3A to 3C, the reason why the heat treatmenttime amounts to 120 seconds in FIG. 3B, longer than the heat treatmenttime of 90 seconds in FIG. 3A is that the number of induction heatingapparatus is small and therefore the transfer speed of the steel productis obliged to decrease for heat treatment under the same temperatureconditions.

Also, although the number of induction heating apparatus is three inFIG. 3C, by making the number of times of passage three, the heattreatment time amounts to 80 seconds, shorter than that of six inductionheating apparatuses and one time of passage in FIG. 3A. This is becausethe transfer speed of the steel product is constant with the number oftimes of passage being one while the heat treatment in case of threetimes of passage can be carried out in a short time by changing thetransfer speed in conformity to the heat treatment. Also, the electricpower consumption rate becomes smaller than in the case where sixinduction heating apparatuses are used.

From the above, it is found that as compared with the case where theheat treatment is carried out in one time of passage with many inductionheating apparatuses being installed, the heat treatment time is shorterand the consumption of electric energy is smaller when the reverseheating is carried out in several times of passage with a suitablenumber of induction heating apparatuses being installed. Besides, it ispossible to reduce the number of induction heating apparatus, which arevery expensive. In the above example, using three induction heatingapparatuses in place of six induction heating apparatuses can reduce thecost to ½ to ⅔. Further, a space necessary for installation is small.

In addition, though not shown here, in the case where the number ofinduction heating apparatus is two and the number of times of passagesis plural, it is possible to considerably reduce the installation costand space although the heat treatment time increases somewhat.

In the case where the heat treatment is carried out in plural times ofpassages with four or five induction heating apparatuses, theinstallation cost and space increase a little but it is possible toconsiderably shorten the heat treatment time.

The reverse heating in plural times of passage is not necessarilyapplied to all the types of steel product but may be applied to the casewhere the heat treatment time is shortened in plural times of passagesand the case where the electric power consumption rate is reduced. Forexample, three times and five times of passage are effective in the casewhere much electric power is needed because of the large dimension ofthe steel product and the large temperature rise. Accordingly, one timeof passage is in some cases advantageous in the case where the dimensionand the temperature rise of the steel product are small. In many cases,plural times of passage are advantageous. For example, FIG. 4 shows thenumber of times of passage to make the heat treatment time advantageousaccording to the dimension and the temperature rise of the steel productwhen the number of induction heating apparatus is three, and FIG. 5shows the number of times of passage to make the electric powerconsumption rate advantageous. However, it is possible to confirm thatone time of passage is in some cases advantageous.

The number of times of passage in the respective embodiments describedabove may be not only odd but also even.

Subsequently, an explanation will be given to embodiments ofmanufacturing facilities of steel product according to the presentinvention.

FIG. 6 shows another example of manufacturing facilities of steelproduct according to the present invention.

The manufacturing facilities comprises, on a hot rolling line, areheating furnace 21, a rolling mill 22, an accelerated coolingapparatus 23, a leveler 24, and an induction heating equipment 25composed of a plurality (here, three) of induction heating apparatuses26. Also, there are attached a transfer speed setting apparatus 28 toset the speed of transfer rollers 27 for transferring the steel product20, an electric power supplying apparatus 29 to supply the electricpower to the respective induction heating apparatus 26, a reheatingfurnace control computer 31 to control the reheating furnace 21, arolling control computer 32 to control the rolling mill 22, a coolingcontrol computer 33 to control the accelerated cooling apparatus 23, anoperating unit 34 to control the induction heating equipment 25, and aproduction control computer 40 to perform the whole production control.Further, a thermometer 30 a is mounted at the outlet side of thereheating furnace 21, a thermometer 30 b is mounted at the outlet sideof the rolling mill 22, a thermometer 30 c is mounted at the outlet sideof the cooling apparatus 23, thermometers 30 d and 30 e are mounted atthe inlet and outlet sides of the leveler 24, and thermometers 30 f to30 k are mounted at the inlet and outlet sides of the induction heatingequipment 25 of each induction heating apparatus 26.

In the manufacturing facilities, the steel product 20 is heated in thereheating furnace 21, then rolled by the rolling mill 22, and thereaftersubjected to accelerated cooling in the accelerated cooling apparatus23. Thereafter, the steel product 20 is corrected in shape by theleveler 24, and then subjected to heat treatment by the inductionheating apparatus 26.

At this time, the reheating furnace control computer 31, the rollingcontrol computer 32, and the cooling control computer 33 track thelocation of the steel product 20, and the tracking data are input intothe operating unit 34. The operating unit 34 performs a predeterminedoperation to determine the number of times of passages and the transferspeed of the steel product in the induction heating equipment 25 and theelectric power for heating, of which results are output to the transferspeed setting apparatus 28 and the electric power supplying apparatus 29to control the induction heating equipment 25.

Details of the reheating furnace control computer 31, the rollingcontrol computer 32, the cooling control computer 33, and the operatingunit 34 will be described as follows with reference to FIGS. 7 to 10.

FIG. 7 shows a detail of the reheating furnace control computer 31 tocontrol the reheating furnace 21.

The reheating furnace control computer 31 comprises an input unit 31 a,an input/output control unit 31 b, a central processing unit 31 c, astorage unit 31 d, and an output unit 31 e. In addition, the storageunit 31 d may comprise any one of stationary magnetic disk, floppy disk,and memory. This is the same with regard to storage units of the othercomputers described later.

FIG. 8 shows a detail of the rolling control computer 32 to control therolling mill 22.

The rolling control computer 32 comprises an input unit 32 a, aninput/output control unit 32 b, a central processing unit 32 c, astorage unit 32 d, and an output unit 32 e.

FIG. 9 shows a detail of the cooling control computer 33 to control thecooling apparatus 23.

The cooling control computer 33 comprises an input unit 33 a, aninput/output control unit 33 b, a central processing unit 33 c, astorage unit 33 d, and an output unit 33 e.

FIG. 10 shows a detail of the operating unit 34 to control the inductionheating equipment 25.

The operating unit 34 comprises an input unit 34 a, an input/outputcontrol unit 34 b, a central processing unit 34 c, a first storage unit34 d, a second storage unit 34 e, a third storage unit 34 f, and anoutput unit 34 g.

First, the reheating furnace control computer 31, the rolling controlcomputer 32, and the cooling control computer 33 receive variousinformation (steel product information) of the steel product 20, whichis treated presently or will be treated from now on, transmitted fromthe production control computer 40, store it into the storage unitsthereof, and set the operating conditions of the reheating furnace 21,the rolling mill 22, and the cooling apparatus 23, which are preset orcalculated, on the basis of dimensions (width, thickness, length)contained in the steel product information, a target reheatingtemperature, and a steel type, in order to perform the followingprocedures.

More specifically, the reheating furnace control computer 31 takes asignal output of the reheating furnace outlet side thermometer 30 a inthe input unit 31 a as shown in, for example, FIG. 7, and makes thecentral processing unit 31 c to monitor temperature through theinput/output control unit 31 b at a constant time period (for example,100 msec). As an example, whether the steel product 20 has beentransferred out of the outlet side of the reheating furnace 21 is judgedon the basis of temperature change per unit time. The time, at which thesteel product 20 has been transferred out of the outlet side of thereheating furnace 2, is recorded as a heating completion time in thestorage unit 31 d and transmitted to the operating unit 34 through theoutput unit 31 e. For time, the function of a timer, which is mountedwithin the reheating furnace control computer 31 to count the presenttime, may be used, and time input from the production control computer40, or time input from an outside may be referred to.

The rolling control computer 32 takes a signal output of the rollingmill outlet side thermometer 30 b in the input unit 32 a as shown in,for example, FIG. 8 and makes the central processing unit 32 c tomonitor temperature through the input/output control unit 32 b at aconstant time period (for example, 100 msec). Whether the steel product20 has been transferred out of the outlet side of the rolling mill 22 isjudged on the basis of temperature change per unit time. The rollingcontrol computer 32 also records the time, at which the steel productleaves the rolling mill 22, as a rolling completion time in the storageunit 32 d and transmits it to the operating unit 34 through the outputunit 32 e. Setting of time is carried out by means of the function of aninternal timer in the same manner as the reheating furnace controlcomputer 31, or by referring to input from the production controlcomputer 40 or an outside.

The cooling control computer 33 takes a signal output of the coolingapparatus outlet side thermometer 30 c in the input unit 33 a as shownin, for example, FIG. 9 and makes the central processing unit 33 c tomonitor temperature through the input/output control unit 33 b at aconstant time period (for example, 100 msec). Whether the steel product20 has been transferred out of the outlet side of the cooling apparatus23 is judged on the basis of temperature change per unit time. Time, atwhich the steel product 20 leaves the cooling apparatus 23, is recordedas a cooling completion time in the storage unit 33 d. Also, the steelproduct information transmitted from the production control computer 40,the heating completion time transmitted from the reheating furnacecontrol computer 31, and the rolling completion time transmitted fromthe rolling control computer 32 are input and recorded in the storageunit 33 d. Then, the steel product information and the coolingcompletion time are transmitted to the operating unit 34 through theoutput unit 33 e. The setting of time is carried out by means of thefunction of an internal timer in the same manner as the reheatingfurnace control computer 31, or by referring to input from theproduction control computer 40 or an outside.

Then, the operating unit 34 forwards the steel product information fromthe production control computer 40, the heating completion time from thereheating furnace control computer 31, the rolling completion time fromthe rolling control computer 32, and the cooling completion time fromthe cooling control computer 33, to the central processing unit 34 cthrough the input unit 34 a and the input/output control unit 34 b torecord them in the first storage unit 34 d. Also, beforehand recorded inthe second storage unit 34 e are a table, in which the number of timesof passage allowed in the induction heating equipment 25 under acombined condition of dimensions and the temperature rise of the steelproduct 20 is set, a plurality of corresponding tables, in which thetransfer speed of the steel product 20 in the induction heatingequipment 25, determined under the combined condition of the dimensionsand the temperature rise of the steel product 20 is set, and a pluralityof tables, in which the power consumption determined by the dimensionsand the temperature rise of the steel product 20 when the number oftimes of passage and the transfer speed are determined is set. Thesetables are referred to in determining the number of times of passage,the transfer speed, and the electric power for heating. Also, recordedin the third storage unit 34 f are the heat treatment pattern calculatedby the operating unit 34 corresponding to the combination of the numberof times of passage allowed under conditions of the steel product, thetransfer speed, and the electric power, and the scheduled time, at whichthe cooling of the succeeding steel product is completed. Then, throughthe operating procedure described later, the operating unit 34determines the number of times of passage; the transfer speed, and theelectric power in the induction heating equipment 25 for the steelproduct 20, outputs the number of times of passage and the transferspeed to the transfer speed setting apparatus 28 from the output unit 34g through the input/output control unit 34 b, and outputs a value ofelectric power for heating to the electric power supply apparatus 29.Here, the heat treatment pattern means a condition, under whichparameters set in the induction heating equipment 25 to subject thesteel product to heat treatment in the induction heating equipment 25 ina manner to obtain desired properties are combined together, and theembodiment adopts a combination of the number of times of passage, thetransfer speed, and the electric power, while parameters, such as setvalues, which change the electric power and the transfer speed accordingto the longitudinal position of the steel product, conditions, underwhich the number of induction heating equipment as used is changed everytime of passage, or the like, having influences on the change in heatingtemperature of the steel product may be added to constitute a heattreatment pattern.

The operating procedure in the operating unit 34 to determine the heattreatment pattern (combination of the number of times of passage, thetransfer speed, and the electric power) will be described below withreference to FIGS. 11 to 14. In the following operations, the number oftimes of passage serves as a reference parameter to determine variousheat treatment patterns, and after the heat treatment patterns are firstdetermined for several numbers of times of passage, the optimum heattreatment parameter such as time, electric power, etc. is selected.

FIG. 11 is a general flowchart of operation.

At a point of time when the operations for the preceding steel productare completed, the operations for the object steel product (a steelproduct to be subjected to heat treatment subsequent to the steelproduct being presently subjected to heat treatment) are started andperformed in the following Step 1 to Step 4.

Step 1: On the basis of the dimensions and the temperature rise, thenumber of times of passage (for example, one, three, five) for possibleheating is determined as a candidate of the number of times of passagefor operations in the next Step with reference to a table of the numberof times of passage for possible heating in the second storage unit 34e.

Step 2: On the basis of the number of times of passages determined inStep 1, the transfer speed and the electric power for heating arecalculated corresponding to respective number of times. As shown in FIG.12, methods of calculating the transfer speed and the electric power forheating include a method of referring to and determining the transferspeed and the electric power for heating on the basis of conditions in apreset table, and a method of calculating the optimal solution in aheating model calculation on the basis of conditions for heat treatment.Accordingly, it is first judged whether the transfer speed is determinedby referring to a table, or through the optimization calculation.Usually, the optimization calculation enabling temperature control withhigh accuracy is selected, while referring to a table is in some casesadopted when the temperature conditions, in which the high accuracy isnot needed, are not strict, and when the steel product havingcomposition, which have not been existing, is to be subjected to heattreatment.

In the case where a table is not referred to, the optimizationcalculation is used to determine the transfer speed and the electricpower for heating and the treatment time is calculated.

On the other hand, in case of referring to a table, the transfer speedis calculated by referring to a table stored in the second storage unit34 e, on the basis of the number of times of passage, the dimensions ofsteel product, and the temperature rise.

Likewise, whether the electric power for heating is determined byreferring to a table, or by means of the optimization calculation isjudged.

In the case where a table is not referred to, after the optimizationcalculation is used to calculate the electric power for heating, theelectric power for heating is determined by calculating the treatmenttime.

On the other hand, in case of referring to a table, the electric powerfor heating is calculated by referring to a table stored in the secondstorage unit 34 e, on the basis of the number of times of passage, thetransfer speed, the dimensions of steel product, and the temperaturerise.

The operation described above is carried out in the number of times ofpassage, which is determined as a candidate in Step 1, for example, inthe case where the number of times being one, three, and five isdetermined as a candidate, so that the operation is respectivelyperformed, namely three times in total, and the transfer speed, theelectric power for heating, and the treatment time are calculated. Here,the calculated results are stored in the third storage unit 34 f.

Step 3: On the basis of the results calculated in Step 2, the optimumnumber of times of passage is determined. As shown in FIG. 13, thecooling apparatus outlet side thermometer 30 c checks whether thecooling of the object steel product is completed. This is because thetime is calculated with timing of leaving the cooling apparatus 23 as areference in order to correctly calculate the time (target treatmenttime) allowable for heat treatment in the induction heating equipment25. The target treatment time is usually set to the time, which preventsthe succeeding steel product from waiting in processes prior to heattreatment process, or the time making a waiting time of the succeedingsteel product shortest when passed the target treatment time. Then, theoperation starts when the object steel product leaves the coolingapparatus 23.

First, the target treatment time for the object steel product iscalculated by acquiring the scheduled time, at which cooling of thesucceeding steel product is completed, and finding a time differencebetween the scheduled time and the time, at which the cooling of theobject steel product is completed. While the target treatment time ishere calculated on the basis of time, at which the cooling is completed,it can also be calculated on the basis of time, at which the steelproduct arrives at the induction heating equipment 25.

Next, it is judged whether the treatment time should have priority.Usually, the shorter treatment time, the less electric power, so thatthe treatment time has priority and that the number of times of passage,in which the heat treatment time becomes the shortest, is selected. Inthe case where the treatment time does not have priority, for example,in the case where the succeeding steel product is delayed and the verylong target treatment time is allowable, the number of times of passage,in which the electric power for heating becomes minimum, is selectedamong conditions, under which the heating is completed within the targettreatment time.

Step 4: Finally, the transfer speed and the electric power for heatingare determined corresponding to the number of times of passagedetermined in Step 3. That is, the heat treatment pattern of theinduction heating equipment 25 is determined thereby.

While the number of times of passage, the transfer speed, and theelectric power are calculated from the dimensions and the temperaturerise of the steel product in the above Step, the steel type can also beadded along with the above.

Subsequently, a method of calculating scheduled time, at which thecooling of the succeeding steel product is completed, described in Step3 will be explained with reference to FIG. 14.

A location of the steel product 20 is tracked by the respectivecomputers 31 to 33. While the tracking method is carried out by outputsof the heating furnace outlet side thermometer 30 a and the rolling milloutlet side thermometer 30 b, it is also possible to use a passagedetection sensor, which makes use of infrared ray, and to make use ofON/OFF of a load on rolling rolls in a rolling mill and a current loadof a motor.

First, the reheating furnace control computer 31 to control thereheating furnace 21 tracks the succeeding steel product to store thetime, at which the succeeding steel product leaves the reheating furnace21, and to transmit such time data to the operating unit 34.

The operating unit 34 calculates the scheduled time, at which thesucceeding steel product leaves the cooling apparatus 23, from thetransfer speed and the transferring distance on the basis of the inputtime data. The calculated scheduled time, at which the cooling of thesucceeding steel product is completed, is stored in the third storageunit 34 f of the operating unit 34.

Further, the rolling control computer 32 to control the rolling mill 22also tracks the succeeding steel product to store the time, at which thesucceeding steel product leaves the rolling mill 22, and transmits thetime data to the operating unit 34.

The operating unit 34 again calculates the scheduled time, at which thesucceeding steel product leaves the cooling apparatus 23, from thetransfer speed and the transferring distance on the basis of the inputtime data. The calculated scheduled time, at which the cooling of thesucceeding steel product is completed, is updated and stored in thethird storage unit 34 f of the operating unit 34. Thereby, it ispossible to more accurately calculate the time, at which the cooling ofthe succeeding steel product is completed. In this case, while thecalculation of scheduled time, at which the cooling is completed, isperformed by the operating unit 34, it can be performed by the heatingfurnace control computer 31, the rolling control computer 32, and thecooling control computer 33, the results of which can be transmitted tothe operating unit 34.

The manufacturing method according to the present invention is not onlyapplicable to the case where the uniform temperature distribution ofsteel plate in the thickness-wise direction is needed but also to thecase where the temperature difference is needed in the thickness-wisedirection.

EXAMPLE

Steel products were subjected to on-line heat treatment using themanufacturing facilities of steel product shown in FIGS. 1 and 2. Here,the induction heating equipment was composed of three solenoid typeinduction heating apparatuses arranged in series. Steel products A and Bwere subjected to accelerated cooling up to 400° C. in the coolingapparatus and steel products C and D were subjected to quenchingtreatment to 100° C. After cooling, tempering treatment was carried outso that the thickness-wise center of the steel product was heated to600° C. The upper limit of the surface temperature of steel product wasset to 720 t corresponding to the Ac1 transformation point of thesesteel products.

TABLE 2 indicates heat treatment time when the steel products A to Dwere subjected to heat treatment with the number of times of passagebeing one and three.

Here, the transfer speed and the electric energy were measured when theheat treatment was carried out with the number of times of passage beingone and three in order to judge which of the number of times of passageshould be selected. According to the result of optimization calculation,it is found that the heat treatment time becomes shorter for the steelproducts A and C with the number of times of passage being one and forthe steel products B and D with the number of times of passage beingthree.

While the tempering treatment was carried out so that the thickness-wisecenter of steel product was heated to a predetermined temperature inthis example, it is possible to use the temperature of an arbitraryposition of steel product as a temperature control.

TABLE 2 A B C D DIMENSIONS OF STEEL MATERIAL 15 × 3500 25 × 3500 15 ×3500 25 × 3500 (THICKNESS × WIDTH mm) TEMPERATURE BEFORE HEATING (° C.)400 400 100 100 HEAT TREATMENT TEMPERATURE (° C.) 600 600 600 600TEMPERATURE RISE (° C.) 200 200 500 500 TRANSFER SPEED WHEN THE NUMBER0.33 0.17 0.17 0.08 OF TIMES OF PASSAGE IS ONE (m/s) ELECTRIC POWER WHENTHE NUMBER 15.7 28.2 33.7 58.7 OF TIMES OF PASSAGE IS ONE (kWh) HEATTREATMENT TIME WHEN THE NUMBER 186.0 324.0 366.0 642.0 OF TIMES OFPASSAGE IS ONE (s) TRANSFER SPEED WHEN THE FIRST TIME 0.67 0.33 0.330.17 NUMBER SECOND TIME 1.17 0.83 0.67 0.5 OF TIMES OF PASSAGE IS THIRDTIME 1.0 0.67 0.5 0.33 THREE (m/s) ELECTRIC POWER WHEN THE NUMBER 22.838.4 40.4 71.0 OF TIMES OF PASSAGE IS THREE (kWh) HEAT TREATMENT TIMEWHEN THE NUMBER 207.4 308.1 396.0 589.0 OF TIMES OF PASSAGE IS THREE (s)

1. A method of manufacturing a steel product comprising heat treating asteel product which has been subjected to quenching or acceleratedcooling on a hot rolling line after hot rolling by passing the steelproduct at least once through a plurality of induction heatingapparatuses, which are installed on the hot rolling line, wherein anumber of times of passage of the steel product through the inductionheating apparatuses is such that a heat treatment time becomes theshortest, the heat treatment time being determined from the number oftimes of passage, a transfer speed of the steel product and an amount ofelectric power for the induction heating apparatuses, and the heattreatment time being the time in which a surface temperature of thesteel product and a thickness-wise center temperature of the steelproduct fall within a predetermined range, wherein a peak value of thesurface temperature of the steel product is controlled so that thesurface temperature does not exceed a maximum value of a predeterminedtemperature range, and wherein conditions in which the heat treatmenttime becomes the shortest are determined by the following steps: (a)determining the dimensions of the steel product and a necessarytemperature rise of the steel product to be subjected to the heattreating, (b) determining the transfer speed and the amount of electricpower for the induction heating apparatuses when the number of times ofpassage is one based on the dimensions and the necessary temperaturerise of the steel product determined in step (a), (c) determining thetransfer speed and the amount of electric power for the inductionheating apparatuses when the number of times of passage is not less thantwo based on the dimensions and the necessary temperature rise of thesteel product determined in step (a), (d) selecting a number of times ofpassage in which the heat treatment time becomes the shortest based onheat treatment times when the number of times of passage is one in step(b) and when the number of times of passage is not less than two in step(c), and (e) determining the transfer speed and the amount of electricpower for the induction heating apparatuses for each passage of thenumber of times of passage selected in step (d).
 2. The method ofmanufacturing a steel product according to claim 1, wherein when theheat treating is carried out when the number of times of passage isthree or more, a transfer speed of the steel product is changed everytime of passage through the induction heating apparatuses.
 3. The methodof manufacturing a steel product according to claim 1, wherein when theheat treating is carried out with the number of times of passage being nwhich is equal to or more than three, transfer speeds of the steelproduct at a nth passage and at a (n-1)th passage are larger than thoseat a (n-2)th passage or before.
 4. The method of manufacturing a steelproduct according to claim 1, wherein the method for determining thetransfer speed of the steel product and the amount of electric power forthe induction heating apparatuses at each of the number of times ofpassage in the steps (b) and (c) comprises solving an optimizationproblem at every time of passage, in which the variables include thetransfer speed of the steel product and the amount of the electric powerfor each induction heating apparatus, constraint conditions include thesurface temperature of the steel product and the center temperature ofthe steel product are within a predetermined range of temperatures, andan objective function includes the heat treatment time and/or the amountof consumed electricity.
 5. The method of manufacturing a steel productaccording to claim 4, wherein the optimization problem in the steps (b)and (c) is solved beforehand and wherein at least one of the transferspeed at every time of passage according to the dimensions of the steelproduct is stored in a table and the amount of electric power of eachinduction heating apparatus is stored in a table.
 6. The method ofmanufacturing a steel product according to claim 1, wherein passing thesteel product at least once through the plurality of induction heatingapparatuses comprises passing the steel product continuously through theplurality of induction heating apparatuses within each pass withoutstopping the steel product.
 7. The method of manufacturing a steelproduct according to claim 1, further comprising the step of determininga possible number of times of passage for the steel product based on thedimensions and the necessary temperature rise of the steel productdetermined in step (a).
 8. The method of manufacturing a steel productaccording to claim 1, further comprising the steps of: determining atarget treatment time based on the time when cooling of the steelproduct is completed or when it arrives at the induction heatingapparatuses and the time when cooling of a succeeding steel product willbe completed, and selecting the number of times of passage in which theheat treatment time becomes the shortest or the number of times ofpassage in which electric power for heating becomes minimum based on thetarget treatment time.
 9. A method of manufacturing a steel productcomprising heat treating a steel product which has been subjected toquenching or accelerated cooling on a hot rolling line after hot rollingby passing the steel product at least once through a plurality ofinduction heating apparatuses, which are installed on the hot rollingline, wherein a number of times of passage of the steel product throughthe induction heating apparatuses is such that a heat treatment timefalls within a target treatment time, the heat treatment time beingdetermined based on the number of times of passage, a transfer speed ofthe steel product and an amount of electric power for the inductionheating apparatuses, and the heat treatment time being the time in whicha surface temperature of the steel product and a thickness-wise centertemperature of the steel product fall within a predetermined temperaturerange, wherein a peak value of the surface temperature of the steelproduct is controlled so that the surface temperature does not exceed amaximum value of a predetermined temperature range, and whereinconditions in which the heat treatment time falls within the targettreatment time are determined by the following steps: (a) determiningthe dimensions of the steel product and a necessary temperature rise ofthe steel product to be subjected to the heat treating, (b) determiningthe transfer speed and the amount of electric power for the inductionheating apparatuses when the number of times of passage is one based onthe dimensions and the necessary temperature rise of the steel productdetermined in step (a), (c) determining the transfer speed and theamount of electric power for the induction heating apparatuses when thenumber of times of passage is not less than two based on the dimensionsand the necessary temperature rise of the steel product determined instep (a), (d) selecting a number of times of passage in which the heattreatment time falls within the target treatment time based on the heattreatment times when the number of times of passage is one in step (b)and when the number of times of passage is not less than two in step(c), and (e) determining the transfer speed and the amount of electricpower for the induction heating apparatuses for each passage of thenumber of times of passage selected in step (d).
 10. The method ofmanufacturing a steel product according to claim 9, wherein the targettreatment time is set to a time which prevents a succeeding steelproduct from waiting in processes prior to the heat treating, or a timewhich results in a waiting time of a succeeding steel product being theshortest when the target treatment time has passed.
 11. The method ofmanufacturing a steel product according to claim 10, wherein the targettreatment time is calculated on the basis of a time at which cooling ofa succeeding steel product is completed, or on the basis of the time atwhich the succeeding steel product arrives at the induction heatingapparatuses.
 12. The method of manufacturing a steel product accordingto claim 10, wherein the number of times of passage through theinduction heating apparatuses is a number which results in a minimumelectric power consumption, among such numbers of times the heattreatment time falls within the target treatment time.
 13. The method ofmanufacturing a steel product according to claim 9, wherein the methodfor determining the transfer speed of the steel product and the amountof electric power for the induction heating apparatuses at each of thenumber of times of passage in the steps (b) and (c) comprise solving anoptimization problem at every time of passage in which variables includethe transfer speed of the steel product and the amount of electric powerfor each induction heating apparatus, constraint conditions include thesurface temperature of the steel product and the center temperature ofthe steel product are within a predetermined range of temperatures, andan objective function includes the heat treatment time and/or the amountof consumed electricity.
 14. The method of manufacturing a steel productaccording to claim 13, wherein the optimization problem in the steps (b)and (c) is solved beforehand and wherein at least one of the transferspeed at every time of passage according to the dimensions of the steelproduct is stored in a table and the amount of electric power of eachinduction heating apparatus is stored in a table.
 15. A method ofmanufacturing a steel product comprising heat treating a steel productwhich has been subjected to quenching or accelerated cooling on a hotrolling line after hot rolling by passing the steel product at leastonce through a plurality of induction heating apparatuses, which areinstalled on the hot rolling line, and a surface temperature of thesteel product and a temperature in a predetermined position inside thesteel product are each determined as a variable based on the number oftimes of passage, a transfer speed of the steel product and an amount ofelectric power for the induction heating apparatuses, wherein the steelproduct is subjected to the heat treating so that a heat treatment timefalls within a target treatment time, the heat treatment time being thetime in which the surface temperature of the steel product does notexceed a predetermined upper limit temperature and the temperature in apredetermined position inside the steel product reaches a targettemperature, wherein a peak value of the surface temperature of thesteel product is controlled so that the surface temperature does notexceed a maximum value of a predetermined temperature range, and whereinconditions in which the heat treatment time falls within the targettreatment time are determined by the following steps: (a) determiningthe dimensions of the steel product and a necessary temperature rise ofthe steel product to be subjected to the heat treating, (b) determiningthe transfer speed and the amount of electric power for the inductionheating apparatuses when the number of times of passage is one based onthe dimensions and the necessary temperature rise of the steel productdetermined in step (a), (c) determining the transfer speed and theamount of electric power for the induction heating apparatuses when thenumber of times of passage is not less than two based on the dimensionsand the necessary temperature rise of the steel product determined instep (a), (d) selecting a number of times of passage in which the heattreatment time falls within the target treatment time based on the heattreatment times when the number of times of passage is one in step (b)and when the number of times of passage is not less than two in step(c), and (e) determining the transfer speed and the amount of electricpower for the induction heating apparatuses for each passage of thenumber of times of passage selected in step (d).
 16. The method ofmanufacturing a steel product according to claim 15, wherein the targettreatment time is set to a time which prevents a succeeding steelproduct from waiting in processes prior to the heat treating, or a timewhich results in a waiting time of a succeeding steel product being theshortest when the target treatment time has passed.
 17. The method ofmanufacturing a steel product according to claim 16, wherein heating ofthe steel product is completed within the target treatment time andperformed so that power consumption is at a minimum.
 18. The method ofmanufacturing a steel product according to claim 16, wherein when theheat treating is carried out when the number of times of passage isthree or more, and a transfer speed of the steel product at a last timeof passage is larger than that at a first time of passage.
 19. Themethod of manufacturing a steel product according to claim 15, whereinthe method for determining the transfer speed of the steel product andthe amount of electric power for the induction heating apparatuses ateach of the number of times of passage in the steps (b) and (c) comprisesolving an optimization problem at every time of passage in whichvariables include the transfer speed of the steel product and the amountof the electric power for each induction heating apparatus, constraintconditions include the surface temperature of the steel product and thecenter temperature of the steel product are within a predetermined rangeof temperatures, and an objective function includes the heat treatmenttime and/or the amount of consumed electricity.
 20. The method ofmanufacturing a steel product according to claim 19, wherein theoptimization problem in the steps (b) and (c) is solved beforehand andwherein at least one of the transfer speed at every time of passageaccording to the dimensions of the steel product is stored in a tableand the amount of electric power of each induction heating apparatus isstored in a table.
 21. A method of manufacturing a steel productcomprising heat treating a steel product which has been subjected toquenching or accelerated cooling on a hot rolling line after hot rollingby passing the steel product at least once through a plurality ofinduction heating apparatuses, which are installed on the hot rollingline, a surface temperature of the steel product and a temperature in apredetermined position inside the steel product are each determined as avariable based on the number of times of passage, a transfer speed ofthe steel product and an amount of electric power for the inductionheating apparatuses, wherein the steel product is subjected to the heattreating so that a heat treatment time becomes the shortest, the heattreatment time being the time in which the surface temperature of thesteel product does not exceed a predetermined upper limit temperatureand the temperature in a predetermined position inside the steel productreaches a target temperature, wherein a peak value of the surfacetemperature of the steel product is controlled so that the surfacetemperature does not exceed a maximum value of a predeterminedtemperature range, and wherein conditions in which the heat treatmenttime becomes the shortest are determined by the following steps: (a)determining the dimension of the steel product and a necessarytemperature rise of the steel product to be subjected to heat treating,(b) determining the transfer speed and the amount of electric power forthe induction heating apparatus when the number of times of passage isone based on the dimensions and the necessary temperature rise of thesteel product determined in step (a), (c) determining the transfer speedand the amount of electric power for the induction heating apparatuseswhen the number of times of passage is not less than two based on thedimensions and the necessary temperature rise of the steel productdetermined in step (a), (d) selecting a number of times of passage inwhich the heat treatment time becomes the shortest based on the heattreatment times when the number of times of passage is one in step (b)and when the number of times of passage is not less than two in step(c), and (e) determining the transfer speed and the amount of electricpower for the induction heating apparatuses for each passage of thenumber of times of passage selected in step (d).
 22. The method ofmanufacturing a steel product according to claim 21, wherein when theheat treating is carried out when the number of times of passage isthree or more, and the transfer speed of the steel product at a lasttime of passage is larger than that at a first time of passage.
 23. Themethod of manufacturing a steel product according to claim 21, whereinthe method for determining the transfer speed of the steel product andthe amount of electric power for the induction heating apparatuses ateach of the number of times of passage in the steps (b) and (c) comprisesolving an optimization problem at every time of passage in whichvariables include the transfer speed of the steel product and the amountof the electric power for each induction heating apparatus, constraintconditions include the surface temperature of the steel product and thecenter temperature of the steel product being within a predeterminedrange of temperatures, and an objective function includes the heattreatment time and/or the amount of consumed electricity.
 24. The methodof manufacturing a steel product according to claim 23, wherein theoptimization problem in the steps (b) and (c) is solved beforehand andwherein at least one of the transfer speed at every time of passageaccording to the dimensions of the steel product is stored in a tableand the amount of electric power of each induction heating apparatus isstored in a table.
 25. A method of manufacturing a steel productcomprising heat treating a steel product which has been subjected toquenching or accelerated cooling on a hot rolling line after hot rollingby passing the steel product at least once through a plurality ofinduction heating apparatuses, which are installed on the hot rollingline, wherein a number of times of passage of the steel product throughthe induction heating apparatuses is such that a heat treatment timebecomes the shortest or falls within a target treatment time, the heattreatment time being determined from the number of times of passage, atransfer speed of the steel product and an amount of electric power forthe induction heating apparatuses, wherein the heat treatment time isdetermined by the following steps: (a) determining the dimensions of thesteel product and a necessary temperature rise of the steel product tobe subjected to the heat treating, (b) determining the transfer speedand the amount of electric power for the induction heating apparatuseswhen the number of times of passage is one based on the dimensions andthe necessary temperature rise of the steel product determined in step(a), (c) determining the transfer speed and the amount of electric powerfor the induction heating apparatuses when the number of times ofpassage is not less than two based on the dimensions and the necessarytemperature rise of the steel product determined in step (a), (d)determining a target treatment time based on the time when cooling ofthe steel product is completed or when it arrives at the inductionheating apparatuses and the time when cooling of a succeeding steelproduct will be completed, (e) selecting the number of times of passagein which the heat treatment time becomes the shortest or the number oftimes of passage in which electric power for heating becomes minimumbased on the target treatment time determined in step (d), and (f)determining the transfer speed and the amount of electric power for theinduction heating apparatuses for each passage of the number of times ofpassage selected in step (e).